In small-sized radio devices, such as mobile phones, the antenna must also be small. For reducing the size of the antenna, already for a long time, the whip of a monopole antenna has been replaced by a coil-like helix conductor or arranged in bends to form a meander pattern, for example. However, for comfort of use, the antenna is by choice placed inside the cover of the device. Internal antennas usually have a planar structure, in which case they include a radiating plane and a ground plane at certain distance therefrom. The space taken by the planar antenna depends on the size of the radiator and its distance from the ground plane. In order to reduce the size of the antenna, this distance can be made very small, but a harmful result of this is the deterioration of the electric characteristics of the antenna. The electric size of the radiator is determined by the operating frequency of the antenna. When the premise is an air-insulated antenna, favorable efficiency-wise, the physical size of the radiator and the size of the whole antenna as well can be reduced by a dielectric substrate. This means that there is a substantially lot of material with relatively high dielectricity between the radiator and the ground plane. The smaller size is achieved at the cost of increased losses of the antenna, but the increase can be kept reasonable by the choosing of materials.
In this description and the claims, an antenna which has a conductor radiator with a dielectric substrate as described above is called a “dielectric antenna”. Numerous of dielectric antennas with somewhat different structures are known. An example of a dielectric antenna according to FIG. 1 is known from the publication EP 0766 340. It includes a circuit board 105 of a radio device, where a part of the upper surface of the circuit board belongs to the conductive ground plane GND. On the circuit board there is a rectangular dielectric piece 121, which functions as the substrate of the radiator. The radiator and its feed and short-circuit have been implemented by coating the substrate 121 with conductive material. The radiator 122 is a strip running across the upper surface of the substrate in its longitudinal direction. The feed conductor of the radiator is formed of a first portion 110 on the surface of the circuit board 105, a second portion 123 on the end surface of the substrate and a third portion 124 on the upper surface of the substrate. The third portion joins to the radiator at a radiator intermediate point. The head end of the radiator 122 is connected to the ground GND through the short-circuit conductor 125 on the above mentioned end surface of the substrate. At its tail end, the radiator can continue to the second end surface of the substrate, not visible in FIG. 2, and extend on it close to the ground on the circuit board 105 for further reducing the size of the antenna. The substrate 121 with its conductive coating forms a unitary antenna component 120, which, as connected to the circuit board, together with it constitutes the whole antenna.
The impedance of the antenna according to FIG. 1 is matched by means of a short-circuit conductor 120 and by choosing the joining point of the feed conductor to the radiator. However, the matching can be unsatisfactory in a part of the operating band of the antenna, in which case it is necessary to use separate matching components, in addition, in the feed circuit of the antenna. An area 150 on the circuit board 105, on which matching components can be connected, is drawn with a dotted line in FIG. 1. Discrete matching components increase the production costs of the antenna. In addition, the reproducibility of the antenna unit is poor because of the tolerances of the component values. This results in a need to tune the antenna, which again has its own practical difficulties.